33
LeakChecks:
TheMS is a powerful tool for determining the presence of leaks in the
GC/MS system because it is able to detect air andwater. Check for leaks by turning on the
PFTBA valve and scanning form/z 69, 18, and 28. By using the base peak for PFTBA (m/z
69), a relative concentration of water/nitrogen can be determined. The combined relative
abundance of 18/28 to 69 should be between 0.1% and 3%. Figure 34 shows an air/water
value of 0.14% (0.05+0.09). If the value is below 0.1%, compare the current total abun-
dance of ion 69with its abundance at the last leak check. Instrumentswith large leaks have
reduced sensitivity for 69 andmay show abundances of 0 for lower ions, suggesting there is
no leak. This is due to saturation of the detector. If a leak is present, the instrument will not
tune.AnMSwith a diffusion pump should be allowedmore time to equilibrate because it is
less efficient at removing lowmolecular weight contamination.After this preliminary
air/water check, begin tuning the instrument.After the instrument passes tuning, check again
for air andwater.
Figure 33.
OptimizeMS tuning to increase sensitivity.
A - Before tune
B - After tune
320000
280000
240000
200000
160000
120000
80000
40000
Abundance
1.00
Time
1.40 1.80 2.20 2.60 3.00 3.40 3.80 4.20 4.60 5.00 5.40 5.80
Figure 34.
An initial air/water check on anAgilent 5971AMS, before tuning.
